This invention pertains generally to impact resistant monovinylidene aromatic copolymer compositions such as those that are commonly known in the art as ABS resins. In particular, said invention pertains to rubber-modified monovinylidene copolymers which can, for example, be prepared by mass, solution or mass/suspension graft copolymerization techniques and which have incorporated (i.e., copolymerized) therein one or more N-substituted maleimide monomers for the purpose of enhancing the heat resistance (e.g., the softening point, heat distortion temperature, etc.) of the resulting graft copolymer product.
Maleimide-modified, impact resistant styrenic copolymer compositions are already known in the art as a general proposition. For example, in U.S. Pat. No. 3,652,726 there are disclosed certain graft copolymers comprising a diene rubber substrate and a superstrate resin comprising acrylonitrile, N-aryl maleimide and an aromatic olefin, such as styrene. Also disclosed are blends of such graft copolymer with various compatible matrix resins. The reference further teaches at col. 6, line 70, that the graft copolymers may be made by a sequential polymerization in which the monomers for the superstrate are polymerized by a free radical process. Bulk, suspension, solution, or emulsion polymerizations are all disclosed as suitable for preparing such polymers. Emulsion techniques are particularly exemplified.
More recently (i.e., in U.S. Pat. No. 4,808,661) there have been disclosed maleimide-modified ABS-type compositions that are prepared by continuous bulk (or mass) polymerization techniques and which are required to meet certain specified criterion and compositional characteristics in order to provide the balance of performance properties which are contemplated for the specific purposes of that patent. In particular, such patent includes the requirements that:
1. the occluded and/or grafted styrene/acrylonitrile/maleimide (S/AN/MI) polymer contained in or on the dispersed rubbery polymer be in the range of 50 to 100 parts per 100 parts of the rubbery polymer; PA1 2. the amount of maleimide in both the grafted/occluded polymer (x) and the continuous matrix phase (y) be in the 1 to 25 weight percent range; PA1 3. the ratio of y:x be greater than 0.5 and less than 2.0; and PA1 4. the crosslinking degree index (hereinafter also referred to as swelling index) thereof be in the range of from 4 to 11. PA1 A. a continuous phase matrix copolymer comprising, in polymerized form and on a matrix copolymer weight basis, from about 35 to about 89 weight percent of a monovinylidene aromatic monomer, from about 10 to about 40 weight percent of an ethylenically unsaturated nitrile monomer, and from about 1 to about 25 weight percent of an N-substituted maleimide monomer; and PA1 B. dispersed within said matrix copolymer, discrete rubber particles having grafted thereon and occluded therein a rigid copolymer constituent comprising, in polymerized form and on a rigid copolymer constituent weight basis, from about 25 to about 94 weight percent of a monovinylidene aromatic monomer, from about 5 to about 40 weight percent of an ethylenically unsaturated nitrile monomer, and from about 1 to about 35 weight percent of a N-substituted maleimide monomer; said rubber-modified monovinylidene aromatic copolymer composition being further characterized in that it has a swelling index value of at least 12 and in that the difference between the N-substituted maleimide monomer content of the matrix phase copolymer and that of the grafted and occluded rigid copolymer constituent is 9 weight percentage points or less. PA1 A. dissolving a rubbery polymer material in a monomer mixture comprising a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer and, optionally, an N-substituted maleimide monomer; PA1 B. partially polymerizing the resulting solution of said rubbery polymer material in said monomer mixture; PA1 C. adding an N-substituted maleimide monomer to the partially polymerized solution of said rubbery polymer material in said monomer mixture when at least 20 weight percent of said monomer mixture has been converted from monomer to polymer; PA1 D. continuing to polymerize the partially polymerized reaction mixture of step (C) to the desired degree of polymerization; and PA1 E. removing any unreacted monomers from the product of step D at elevated temperature and reduced pressure and under conditions such that the swelling index of the resulting rubber modified polymer product is 12 or greater. PA1 1. Add 30 ml of a 70/30 volume ratio mixture of methyl ethyl ketone (MEK)/methanol to 1.0 gm of the resin to be analyzed and shake the mixture for at least 6 hours. PA1 2. Centrifuge at 19,500 RPM and 5.degree. C. for 2 hours. PA1 3. Pour off the supernatant. PA1 4. Dry the remaining gel phase at 150.degree. C. and ambient pressure for 30 minutes, and at 150.degree. C. and 5 mm Hg pressure (absolute) for 60 minutes. ##EQU2## 5. Elemental analysis of the gel phase for oxygen content will then allow determination of the percent of N-substituted maleimide monomer contained in the graft and occlusions.